1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal-combustion engine by means of a specific component in the exhaust gas.
2. Description of the Prior Art
A prior art feedback (closed loop) controlling method for controlling the air-fuel ratio repeats the following steps so as to control the center value of the air-fuel ratio within a narrow range of air-fuel ratios around the stoichiometric ratio required for a three-way reducing and oxidizing catalytic converter. First, the intake-air amount (or the intake-air pressure) and the running speed of the engine are detected. Then, a base-fuel injection amount to be supplied into fuel injectors are calculated depending upon the detected intake-air amount and the detected engine speed. The base-fuel injection amount is corrected by using an air-fuel compensation factor which is calculated from a detection signal of a concentration sensor (O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen concentration in the exhaust gas of the engine. Thus, the corrected fuel-injection amount determines the actual fuel-feeding rate of the engine. Therefore, since the air-fuel ratio is controlled within a very small range around the stoichiometric ratio, the catalytic converter can maintain its capability at a high level so as to effectively remove three gas-containing pollutants, i.e., carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxide (NO.sub.x).
In the above-mentioned air-fuel ratio feedback control method, however, in order to maintain the cleaning capacity of the catalytic converter at a maximum, the air-fuel ratio is controlled depending upon the indication signal of the O.sub.2 sensor so as to attain .lambda. (air ratio)=1, even during the deceleration mode or the idle mode. As a result, the air-fuel ratio may fluctuate during the deceleration mode, and the air-fuel ratio may shift a little to the rich side during the idle mode. In this case, the atmosphere within the catalytic converter is reduced so that the converter gives out odorous fumes, such as hydrogen sulfide gas, in the exhaust gas.
In order to solve the above-mentioned problem, one approach is to control the air-fuel ratio to satisfy .lambda.&gt;1 indiscriminately during the deceleration mode and the idle mode, which, however, tends to reduce the driving characteristics, make the engine speed unstable, reduce the capacity of the catalytic converter to clean the emission gas, etc.